In piston cylinder units of this type there is a risk when a pressure in a compression cavity is greater than a pressure in a bearing gap that the piston is laterally tilted from a position that is coaxial with the cylinder wherein the tilting is caused by fluid from the compression cavity that enters the bearing gap asymmetrically. This changes a thickness of the bearing gap at least partially and rapidly reduces a load bearing capability of the fluid pressure bearing between the cylinder and the piston. In particular when the piston-cylinder unit is configured as a compressor tilt stability for the piston has to be provided.
DE 10 2004 061 904 A1, whose disclosure is incorporated in its entirety by this reference discloses a piston-cylinder unit which provides increased tilting stability for the piston. This known piston cylinder unit is illustrated in FIG. 1 as prior art. This figure illustrates a longitudinal sectional view through a piston cylinder unit 1 with a cylinder 2 and a piston 3. The cylinder is provided with a cylinder bore hole 10 in which the piston 3 is moveable back and forth in a direction of a longitudinal axis X of the cylinder bore hole 10 and received freely supported. The piston 3 is connected through a piston rod 4 with an input or an output which are not illustrated. The cylinder face wall 12 that is configured at a cylinder head 23 and which forms the face side termination of the cylinder bore hole 10, the inner circumferential wall of the cylinder bore hole 10 and the piston face wall 16 define the cylinder volume and define a pressure cavity 18.
An inlet channel 22 provided with a schematically illustrated valve 20 leads into the cylinder face wall 12. An outlet channel 24 is also provided in the cylinder face wall 12, wherein the outlet channel 24 also includes a respective outlet valve 26. Also this outlet channel leads into the cylinder bore hole 10.
When the piston is moved in FIG. 1 to the right up to the dashed position of the piston 3 where it reaches its top dead center TDC where it reverses its movement direction, the fluid that is included in the cylinder volume 18 and which is for example gaseous is compressed when the piston-cylinder unit is a compressor. The cylinder volume 18 then forms a compression cavity. When the outlet valve 26 opens the compressed fluid flows out of the compression cavity 18 through the outlet channel 24, for example to downstream consumers.
A portion of the expelled fluid is conducted out of the outlet channel 24 through a connection channel 28 that is provided in the cylinder head 23 and in the housing 21 of the cylinder 2 into ring channels 30, 32, 34 which are also provided in the housing 21 of the cylinder 2 and which envelop the cylinder bore hole 10 in an annular manner. The ring channels 30, 32, 34 are offset from one another in a direction of the longitudinal axis X of the cylinder bore hole. Each of the ring channels 32, 33, 34 is provided with a plurality of micro holes 30′, 32′, 34′ which are evenly distributed over a circumference of the cylinder bore hole 10 and respectively connect the ring channel 32, 33, 34 with an interior of the cylinder bore hole 10 and thus penetrate the inner circumferential wall 14 of the cylinder. The micro holes 30′ 32′, 34′ of each ring channel 30, 32, 34 thus respectively form an annular nozzle arrangement 30″, 32″ 34″. Pressurized fluid, advantageously pressurized gas which is conducted through the connection channel 28 into the ring channels 30, 32, 34 can thus exit through the micro holes 30′, 32′ 34′ and can form a fluid cushion for example a gas cushion laterally supporting the piston in a bearing gap 19 between a cylinder side bearing surface 15 on an inner circumferential wall 14 of the cylinder 2 and a piston side support surface 38 on an outer circumferential wall of the piston.
The first ring channel 30 that is most proximal to the cylinder face wall 12 and includes associated micro holes 30′ is arranged in a portion in which the piston only covers the micro holes 30′ only when the piston is arranged proximal to the compression position, thus the top dead center, thus when the cylinder volume 18 is minimized. In this case the piston 3 covers the forward first micro holes 30′ with the bearing surface 38 in a front portion 3″. This way it is assured that the piston section which is adjacent to the piston face wall 16 is laterally stabilized in its position proximal to the top dead center TDC, so that a risk that the piston is laterally displaced by fluid entering the bearing gap from the compression cavity is essentially excluded.
The second ring channel 22 is arranged so that micro holes 32′ associated therewith are always covered by the moving piston 3 so that the micro holes 32′ help to form the supporting gas cushion between the inner circumferential wall 14 of the cylinder 2 and the outer circumferential wall 36 of the piston 3 over an entire axial movement path of the piston 3.
The third ring channel 34 is the furthest away from the cylinder face wall 12. The micro holes 34′ associated with the third ring channel 34 are thus covered by the piston 3, thus by the support surface 38 in the rear portion 3′ of the piston only when the piston 3 is in its retracted position in which the cylinder volume is at a maximum.
This known piston-cylinder unit supports the piston in its forward circumferential portion also in its top dead center position but it cannot be excluded that pressurized fluid that enters the bearing gap from the compression cavity 18 imparts a lateral force upon the piston because a distance between the piston face wall and the impact location of the pressurized bearing fluid exiting from the micro holes 30′ varies at the piston circumference due to the piston movement.
DE 10 2008 007 661 A1 shows a linear compressor with a piston-cylinder unit whose piston is driven by a linear motor to perform a reciprocating movement. The piston is gas pressure supported in the cylinder and the cylinder wall is provided with a plurality of nozzle openings for this purpose. The piston is provided with a plurality of slanted bore holes or radial slots at its face side, wherein the slanted bore holes or radial slots extend from a base of the piston to a circumference of the piston. A pressure balancing between the spaces on both sides of the piston shall be provided through the bore holes or slots.
From DE 81 32 123 U1 a gas pressure support of a piston-cylinder unit is known, wherein a fluid connection is provided between the compression cavity and a pressure cavity of the gas bearing.
U.S. Pat. No. 5,140,915 A illustrates and describes a gas supported piston in a piston-cylinder unit in which circumferential grooves are provided in the forward end section wherein the circumferential grooves are introduced into the circumferential wall in an insulated manner. These circumferential grooves are configured to insulate the gas bearing from an oscillating pressure in the compression cavity.
JP 2002 349 435 A discloses a linear compressor with an air supported piston which is provided with a circumferential groove in its center section in axial direction. This circumferential groove provides pressure compensation along the circumference of the piston and thus pressure compensation acting in circumferential direction in the bearing gap. When compressed air moves in this known linear compressor from the compression cavity into the bearing gap at a location of the bearing gap, forces which might cause a tilting of the piston are quickly compensated by the pressure compensation caused by the circumferential groove, so that the piston quickly moves back into its position that is coaxial with the cylinder axis or in an ideal case it does not even leave this position. This circumferential groove does not only weaken the undesirable transversal force, but also the air bearing which reduces load bearing capability of the air bearing.
A piston cylinder unit is known from U.S. Pat. No. 2,907,304 which forms a linear drive device that is actuatable by a fluid. The cylinder wall is provided with a plurality of openings through which a pressurized fluid is introduced into the cylinder cavity. Furthermore switchable fluid outlets are provided at both face sides of the cylinder housing so that alternating opening of respective outlet valves generates a linear movement of the piston.